FIG. 1 shows the general configuration of a drilling system in a Measurement-While-Drilling (MWD) or Logging-While-Drilling (LWD) environment. A downhole tool 10 disposes in a borehole BH and is operationally connected to a drill string 12 by a suitable connector 14. At its lower end, the tool 10 has a drill bit 16. Uphole, a rotary drilling rig 60 rotates the drill string 12, the downhole tool 10, and the drill bit 16 to drill the borehole BH. As will be appreciated, other types of borehole conveyance can be used for the downhole tool 10.
The downhole tool 10 has a drill collar 20, a borehole sensor 50, and an electronics subsection 52. The drill collar 20 has a stabilizer sleeve 30 disposed thereon, and the borehole sensor 50 is mounted at a stabilizer blade 32. Depending on the desired parameters of interest, the borehole sensor 50 measures data in the borehole environs, and the electronics subsection 52 can process and store the data and can telemeter the data uphole for any of the various purposes associated with LWD/MWD.
A surface processor 64 cooperating with the electronic subsection 52 may handle the data and can perform additional mathematical operations associated with standard geological applications. Processed data can then be output to a recorder 66 for storage and optionally for output as a function of measured depth thereby forming an “image” or “log” 68 of one or more parameters of interest. All throughout operations, signals can be sent downhole to vary the direction of drilling or to vary the operation of the downhole tool 10.
There are a few techniques for mounting a sensor on a downhole tool 10 for interaction with a borehole BH. Conventional wisdom in the art has been to either install the sensor externally on a drill collar or stabilizer or to particularly configure the sensor to install on the drill collar or stabilizer. Thus, one technique simply mounts a sensor with a plate on a portion of a drill collar. For example, U.S. Pat. No. 7,250,768 to Ritter et al. discloses a modular cross-over sub for a bottom hole drilling assembly having a stabilizer. Separate from the stabilizer, a resistivity sensor on a plate affixes to the outside of the sub where the sensor and measuring electronics are disposed.
Alternatively, a sensor can be directly part of a stabilizer. For example, U.S. Pat. Pub. No. 2009/0025982 discloses instrumentation devices disposed externally on a blade of a stabilizer using rings attached to the blade with screws or other attachment means.
Finally, a particularized package for a sensor can fit in a recess of a downhole tool and can have a stabilizer fit thereover. For example, U.S. Pat. No. 6,666,285 to Jones et al. discloses a drilling conduit having a cavity particularly sized to receive an instrument package. A portion of the package radially protrudes a distance, and an alignment channel in a stabilizer element is dimensioned to receive the protruding portion of the instrument package. For ease of manufacturing, the alignment channel extends the entire length of the stabilizer element.
As a particular example, FIG. 2 is a side cross-section of a portion of a downhole tool 10 having a sensor and stabilizer arrangement according to the prior art. The drill collar 20 is shown with its internal bore 22 for passage of drilling fluid. A sensor housing 40 fits inside a recess or pocket 24 formed on the outside surface 23 of the drill collar 20 and hard-mounts to the drill collar 20 using mounting components 42. The sensor housing 40 has a sensor 50 (e.g., LWD downhole measurement equipment), and the hard mounting of the housing 40 provides stable positioning of the sensor 50 and helps protect the sensor 50 from damage.
The sensors used for LWD/MWD applications typically measure parameters of the formation traversed by the borehole or of the borehole itself. In typical applications, measurement accuracy is degraded by excessive and/or inconsistent standoff between the sensor and the surrounding borehole wall. To reduce standoff, the sensor 50 may actually be positioned in the drill collar's pocket 24 at a further radial distance than the drill collar's outer surface 23. This allows the sensor 50 to position closer to the borehole wall. To help maintain the consistent standoff and to protect the sensor 50, a stabilizer sleeve 30 is typically employed and is positioned directly on the drill collar's outer surface 23. When the sleeve 30 is pushed into position on the outside of the drill collar 20, one of the stabilizer blades 32 on the stabilizer sleeve 30 fits directly over the sensor housing 40, and the stabilizer sleeve 30 can be retained using a shoulder on the drill collar 20 and a bushing 34 or other features.
Because the housing 40 is physically mounted to the collar 20, the distance between the sensor 50 and the borehole wall will change if the diameter of the borehole BH to be drilled is changed and if the stabilizer sleeve's diameter is also changed accordingly. This impacts the ability to make consistent measurements with the sensor 50 when used in different configurations because the changes in distance from the borehole wall will attenuate the measurements made.
For example, FIGS. 3A-3B are end views diagramming the prior art sensor and stabilizer arrangement for different sized boreholes BH1 and BH2. As can be seen, the radius R1 of the first borehole BH1 is smaller than the radius R2 of the second borehole BH2. As is common, the same sized drill collar 20 may be used to drill both of these boreholes BH1 and BH2, while other components of the drilling system are changed to create the different sized boreholes BH1 and BH2. To account for the difference in borehole size relative to the same sized drill collar 20, different sized stabilizer sleeves 301 and 302 are used when drilling. For instance, the first stabilizer sleeve 301 for the smaller borehole BH1 has lower profile stabilizer blades 321, while the other stabilizer sleeve 302 for the larger borehole BH2 has higher profile stabilizer blades 322.
Yet, in both circumstances, the sensor housing 40 hard-mounted to the drill collar 20 keeps the sensor 50 at the same position on the drill collar 20. As a result, the sensor 50 has a smaller standoff S1 relative to the wall of the smaller borehole BH1, but has a larger standoff S2 relative to the wall of the larger borehole BH2.
For measurement accuracy, the sensor 50 is typically calibrated electronically and with processing algorithms to operate best with a particular standoff from the borehole wall. Due to the different sized stabilizer sleeves 301 and 302 needed in some drilling applications as seen in FIGS. 3A-3B, the standoff under which the sensor 50 measures can change. To obtain useful measurements, operators must therefore recalibrate the sensor 50 to operate with the different standoffs S1 and S2, or an entirely different sensor housing 40 may need to be used so the sensor 50 will have the calibrated standoff.
As always, changes or modifications made in drilling applications can increase costs, slow down drilling operations, engender unwanted errors, and the like. For these and other reasons, the subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.